[Bug c++/84796] New: ICE in a template parameter pack expansion

[v.reshetnikov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84796

            Bug ID: 84796
           Summary: ICE in a template parameter pack expansion
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: v.reshetnikov at gmail dot com
  Target Milestone: ---

The following C++ code causes an ICE in GCC 8.0.1 20180308 (tested with
https://godbolt.org/):

/************* SOURCE *************/
template<typename... T>
struct S {
    template<T... n>
    static void f() { }
};

int main(){
    S<int>::f<0>();
}
/*********** END SOURCE ***********/


/************* OUTPUT *************/
<source>: In function 'int main()':
<source>:8:18: internal compiler error: tree check: accessed elt 1 of tree_vec
with 0 elts in tsubst_pack_expansion, at cp/pt.c:11743
     S<int>::f<0>();
                  ^
mmap: Invalid argument
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.
Compiler returned: 1
/*********** END OUTPUT ***********/

Looks like a regression: no ICE occurs with GCC 7.3, but the code is rejected
with an error (I believe, incorrectly):

/************* OUTPUT *************/
<source>: In function 'int main()':
<source>:8:18: error: no matching function for call to 'S<int>::f<0>()'
     S<int>::f<0>();
                  ^
<source>:4:17: note: candidate: template<T ...n> static void S<T>::f() [with T
...n = {n ...}; T = {int}]
     static void f() { }
                 ^
<source>:4:17: note:   template argument deduction/substitution failed:
Compiler returned: 1
/*********** END OUTPUT ***********/